get rid of using `__builtin_unreachable` directly [Bug #17787]

	As it is an independent feature from `clz`, `ctz` and `popcount`,
	it might be unavailable even if the latters are built in.
	---
	 internal/bits.h | 6 +++---
	 1 file changed, 3 insertions(+), 3 deletions(-)